1. Field of the Invention
The present invention relates to an energy filter consisting of at least one electron-deflecting magnet assembly to pass incident electrons which have a certain energy.
2. Description of the Related Art
Electron microscopes having electron optics incorporating an energy filter have been developed. Such a conventional electron microscope is shown in FIG. 6, in which the microscope is indicated by numeral 1. This microscope has an electron gun 2 emitting a beam of electrons e. The beam is directed to a specimen 5 via a condenser lens system 3. The beam transmitted through the specimen 5 is projected onto a fluorescent screen 11 via an objective lens 4, an intermediate lens 6, an entrance aperture 7, a spectrometer 8, a slit 9, and a projector lens 10. Thus, a transmission image of the specimen is observed. The entrance aperture 7, the spectrometer 8, and the slit 9 constitute an energy filter 12, known as an xcexa9-filter.
The spectrometer 8 incorporated in the energy filter 12 of the electron microscope 1 is equipped with at least one electron-deflecting magnet assembly. One example of the electron-deflecting magnet assembly is shown in FIGS. 7(a) and 7(b), where the magnet assembly, indicated by 13, comprises a pair of opposed magnetic polepiece bases 100, 101. Coil grooves 22 and 24 are formed adjacent to each other in one surface of the magnetic polepiece base 100. Thus, those portions which are surrounded by the coil grooves 22 and 24 form magnetic polepieces 14 and 16, respectively. Coils 18 and 20 are received in the coil grooves 22 and 24, respectively. Similarly, the other magnetic polepiece base 101 is provided with coil grooves 23 and 25 formed adjacent to each other. Thus, those portions which are surrounded by the coil grooves 22 and 24 form polepieces 15 and 17, respectively. Coils 19 and 21 are received in the coil grooves 23 and 25, respectively. The polepiece bases 100 and 101 are so positioned that the formed polepieces 14 and 16 are located opposite to the polepieces 15 and 17, respectively. Those portions of the polepieces 14-17 that are surrounded by the coil grooves 22-25 are recessed as viewed from the other portions. Gaps 26 and 27 are formed between them and in communication with each other via a passage 28. These gaps 26, 27, and passage 28 together form an electron passage 29.
Electrical current is supplied from a current source (not shown) to the coils 18-21 to produce magnetic fields in the gaps 26 and 27 between the polepieces 14 and 15 and between the polepieces 16 and 17, respectively. Shunts (not shown) are mounted at the entrance and exit surfaces of the gaps 26 and 27 to prevent ooze or spreading of the magnetic fields. Using these shunts, the distributions of the magnetic fields developed in the gaps 26 and 27 between the polepieces are tightly controlled. Electrons are caused to pass through these magnetic fields. This gives good electron optical characteristics to the electron-deflecting magnet assembly 13 acting to deflect electrons.
Electrons react with molecules within air and are lost rapidly. It is necessary to evacuate the coil grooves 22-25 and the electron passage 29 within the electron-deflection magnet assembly 13 to create a low-pressure condition. In the past, therefore, the electron-deflecting magnet assembly 13 itself has been accommodated within a vacuum chamber. With this method for evacuating the electron-deflecting magnet assembly 13, however, it is very difficult to pump down the inside of the magnet assembly 13 because the components of the magnet assembly 13, such as the coils 18-21, have large surface areas. Where there is a large amount of residual gas, the electron microscope 1 fitted with the energy filter 12 suffers from various problems, such as instability of the accelerating voltage and specimen contamination due to electron irradiation.
In an attempt to solve these problems, the following two methods have been adopted. A first method consists of placing a tube 30 along an electron passage 29 as shown in FIG. 8 and evacuating only the inside of the tube 30. With this first method, it can be expected that the aforementioned problems will be solved at the highest efficiency, since the volume evacuated is smallest.
A second method consists of covering the coils 18-21 with vacuum-resistant packs 31-34, respectively, as shown in FIG. 9, to suppress degassing from the coils 18-21. With this second method, intrusion of gas into the electron passage 29 is suppressed, the gas escaping from the coils 18-21. Therefore, the aforementioned problems can be effectively solved.
With the first method, it is necessary to accurately shape the tube 30. Since the tube 30 is very complex in shape, it is very difficult to shape the tube 30 accurately. Furthermore, it is necessary to clean the inside of the tube 30. However, it is not easy to finish the interior of the tube 30 with a high degree of cleanliness.
To put the tube 30 in the electron passage 29, the gaps 26 and 27 between the polepieces 14 and 15 and between the polepieces 16 and 17 are inevitably set large. If these gaps are made large, a larger power supply is necessary to produce a given magnitude of magnetic field. In addition, the aberrations of the deflecting magnetic field increase. Accordingly, limitations are imposed on increase of the gaps 26 and 27.
In the second method described above, the coils 18-21 are separately covered with the vacuum-resistant packs 31-34, respectively. Therefore, the coil grooves 22-25 in the coils 18-21 must have large space. This increases the size and complexity of the electron-deflecting magnet assembly 13. Additionally, the gap between each shunt and the corresponding polepieces, such as 14-17, is increased to secure spaces to accommodate the coils 18-21.
In view of the foregoing, the present invention has been made.
It is an object of the present invention to provide an energy filter that can be designed compactly without increasing the gaps between polepieces or spaces to accommodate coils and has an electron passage capable of being evacuated more reliably.
An energy filter built in accordance with a first embodiment of the present invention solves the foregoing problems and comprises at least one magnet assembly mounted in a vacuum created within an electron microscope, the magnet assembly being designed to pass only incident electrons which have a certain energy. The magnet assembly comprises a pair of polepiece bases located opposite to each other, polepieces and coil grooves formed in respective surfaces of the polepiece bases, coils inserted in the coil grooves, respectively, a pair of spacers interposed between the polepiece bases, and a yoke fixedly mounted to side surfaces of the polepiece bases. The coil grooves are located opposite to each other. The spacers are provided with sealing grooves to accommodate hermetic seals, respectively, for hermetically sealing the coils received in the coil grooves in the opposite polepiece bases, respectively. At least one electron passage gap is between the spacers to form an electron passage. Seal members are inserted in the sealing grooves, respectively, to permit the coils to be located outside the vacuum described above.
An energy filter in accordance with a second embodiment of the present invention is based on the energy filter in accordance with the first embodiment and further characterized in that the polepieces have bulges swelling outward from the coil grooves, respectively, to form shunts for preventing ooze of magnetic fields.
An energy filter in accordance with a third embodiment of the present invention is based on the energy filter in accordance with the first or second embodiment and further characterized in that at least one magnet assembly described above is plural and fixedly mounted to a platen.
An energy filter in accordance with a fourth embodiment of the invention is based on the energy filter in accordance with the first or second embodiment and has the following features. The aforementioned at least one magnet assembly is plural. The magnet assemblies include first magnet assemblies having a pair of polepiece bases which are integrally fabricated, respectively, and a pair of spacers which are integrally fabricated, respectively. The electron passage gap is formed between the integrally fabricated spacers. Sealing grooves are formed in the integrally fabricated spacers, respectively. Sealing members are received in the sealing grooves, respectively.
In the energy filter constructed as described above, the sealing members permit the coils of the magnets to be located outside the vacuum, and the coils are not brought within the vacuum. This prevents deterioration of the vacuum inside the electron microscope. Hence, the performance of the electron microscope is prevented from deteriorating.
The electron passage gap formed between the spacers form a single electron passage. This passage is much easier to machine and clean than the conventional tube described above.
Since no tube is accommodated within the electron passage, it is not necessary to secure a large space between the opposite polepieces. In consequence, the magnet assembly can be designed compactly. Furthermore, only a small-size power supply suffices.
The shunts for preventing ooze of the magnetic fields are formed integrally with the polepieces at the bulges of the polepieces. Therefore, it is not necessary to take account of the accuracy with which these components are assembled. The magnet assembly can be assembled easily.
Other objects and features of the invention will appear in the course of the description thereof, which follows.